Automatic welding filters are well known, and generally consist of a switchable filter being located on, or as part of, personal protective equipment (e.g., headwear or eyewear). The switchable filter is able to automatically switch from a light transmission state to a dark transmission state in response to activation by welding light. This is generally achieved by using a sensor located on, or as part of, personal protective equipment that detects the start of a welding arc and generates a corresponding control voltage which, when applied to the switchable filter, causes it to change from a light transmission state to a dark transmission state. Activation of the switchable filter can also be controlled by transmitting activation signals along a communication channel between a welding torch and the automatic welding filter, which is controlled by a corresponding communication unit, see WO2007/047264 (Garbergs, et al.). This technique ensures that the welding tool is not activated before the switchable filter has reached its dark transmission state.
U.S. Pat. No. 4,240,709 (Hornell) describes that a switchable filter can be formed by sandwiching a single twisted nematic liquid crystal cell between a pair of mutually crossed polarisers; however, the majority of commercial products now utilise a switchable filter formed by sandwiching two twisted nematic liquid crystal cells between three crossed polarisers. The liquid crystal molecules are able to orientate when a voltage is applied across the liquid crystal cells, under the control of an electronic module. The switchable filter is then automatically darkened by the electronic module, upon detection of incident welding light that falls on a photodetector. Prior art automatic welding filters also include an interference filter which attenuates the harmful IR and UV wavelength components of the welding light, and a third liquid crystal cell can sometimes be included (usually a guest-host type liquid crystal cell) which provides a “fail-safe” intermediate transmission state in the case of failure of the electronic module. WO95/29428 (Hornell, et al.) discloses such a prior art automatic welding filter.
The invention described in WO95/29428 also addresses the problem that, in the dark transmission state, the switchable filter is darkened unevenly due to the fact that the filter effect is heavily dependent upon the angle of incidence of the welding light. WO95/29428 proposes a switchable filter including low twisted nematic liquid crystal cells, where the twist angle differs from the conventional 90 degrees, and is less than 85 degrees. This approach gives significant improvements in terms of homogeneity in the dark transmission state of the switchable filter.
The use of such an automatic welding filter can greatly increase the accuracy of electrode placement, giving higher quality welds. Productivity is also increased as the need for grinding and rework is correspondingly reduced. However, with existing automatic welding filters, the emission spectrum of welding light varies depending on the welding method and materials used, and this can sometimes make viewing the welding task more difficult in the dark transmission state and which can otherwise lead to a reduction in efficiency and weld quality.